Raise a more informative error when scenario variant is missing a variable key

[tomalrussell]

e.g. in a scenario definition, for a scenario which should provide multiple outputs:

variants:
  - name: es_energy_demand_dummy_full
    description: ''
    data:
        a: file.csv
        # oil_non_heat_eh not included here

trying to access (through DataHandle.get_data) an output which has no entry in the variant data dict raises a KeyError

  File "/home/vagrant/nismod/lib/python3.5/site-packages/smif/data_layer/data_handle.py", line 304, in get_data
    data = self._get_scenario(dep, timestep, input_name)
  File "/home/vagrant/nismod/lib/python3.5/site-packages/smif/data_layer/data_handle.py", line 383, in _get_scenario
    timestep
  File "/home/vagrant/nismod/lib/python3.5/site-packages/smif/data_layer/store.py", line 546, in read_scenario_variant_data
    key = self._key_from_data(variant['data'][variable], scenario_name, variant_name,
  File "/home/vagrant/nismod/lib/python3.5/site-packages/ruamel/yaml/comments.py", line 716, in __getitem__
    return ordereddict.__getitem__(self, key)
KeyError: 'oil_non_heat_eh'